Generally, in order to enhance the efficiency of an amorphous solar cell, an amorphous SiGe film is used as a material which enables effective utilization of long wavelength light which cannot be absorbed by a film including only amorphous Si.
FIG. 3 shows a cross-sectional view of a pin type single element amorphous SiGe solar cell produced on a stainless steel substrate. A cell comprising a P type amorphous Si (or a P type microcrystalline Si) film 2, a non-doped amorphous SiGe film 3, and an n type amphorous Si (or an n type microcrystalline Si) film 4 is produced on the stainless steel substrate 1. A transparent conductive film 5 and a metal electrode 6 are also formed on the cell.
In order to enhance the efficiency, it is rare that an amorphous SiGe cell is used singly as described above, and it is usually used as a bottom cell of a multicell structure as shown in FIG. 4. In FIG. 4, an amorphous SiGe cell is constituted by layers 2, 3, and 4, and cells comprising high band gap material such as amorphous Si are constituted by layers 2, 8, and 4 and 2, 7, and 4.
Conventionally, an amorphous SiGe film having good properties is produced by a plasma CVD method using a mixed gas of SiH.sub.4 and GeH.sub.4, or a mixed gas obtained by diluting SiH.sub.4 and GeH.sub.4 with e.g. H.sub.2 or He. When the ratio of the flow rate of GeH.sub.4 gas to the total gas flow rate is increased, the Ge concentration in the film increases, and the band gap thereof is decreased continuously from about 1.8 eV of amorphous Si. Although the response to long wavelength light then increases, the carrier mobility is decreased with the increase in the Ge concentration. Thus, a practical amorphous SiGe film requires a band gap of above 1.5 eV.
Furthermore, when the film is produced from a mixed gas of SiH.sub.4 and GeH.sub.4, a high photoconductivity and good film properties are obtained under growth conditions of a relatively low gas pressure and a high RF power. A pin type of an nip type amorphous SiGe solar cell using such a film for a non-doped layer has a response for long wavelength of up to about 800 nm, and a conversion efficiency of about 11% is obtained in a multilayer structure cell including that cell together with a cell using an amorphous Si layer. A further enhancement in the conversion efficiency is expected by an enhancement in the film quality of the amorphous SiGe film. A theoretical conversion efficiency of a triple layer structure element is calculated as 19 to 24%.
On the other hand, the light photoconductivity of an amorphous Si film is reduced by light irradiation. This is the so called Staebler-Wronski effect. The conversion efficiency of a solar cell is also reduced by a long period of light irradiation, and this causes a problem in its reliability. The same effect arises in an amorphous SiGe film, and the solar cell characteristics also deteriorate because of a light irradiation to a degree dependent on the Ge concentration in the film.
Another problem in an amorphous SiGe film produced by the conventional method resides in the fact that exfoliation of the film is likely to arise due to a large internal stress generated in the amorphous SiGe film when the film thickness is increased.